Gaps in the ablation line as a potential cause of recovery from electrical isolation and their visualization using MRI

Abstract

Background: Ablation has become an important tool in treating atrial fibrillation and ventricular tachycardia, yet the recurrence rates remain high. It is well established that ablation lines can be discontinuous and that conduction through the gaps in ablation lines can be affected by tissue heating. In this study, we looked at the effect of tissue conductivity and propagation of electric wave fronts across ablation lines with gaps, using both simulations and an animal model.

Methods and results: For the simulations, we implemented a 2-dimensional bidomain model of the cardiac syncytium, simulating ablation lines with gaps of varying lengths, conductivity, and orientation. For the animal model, transmural ablation lines with a gap were created in 7 mongrel dogs. The gap length was progressively decreased until there was conduction block. The ablation line with a gap was then imaged using MRI and was correlated with histology. With normal conductivity in the gap and the ablation line oriented parallel to the fiber direction, the simulation predicted that the maximum gap length that exhibited conduction block was 1.4 mm. As the conductivity was decreased, the maximum gap length with conduction block increased substantially, that is, with a conductivity of 67% of normal, the maximum gap length with conduction block increased to 4 mm. In the canine studies, the maximum gap length that displayed conduction block acutely as measured by gross pathology correlated well (R(2) of 0.81) with that measured by MRI.

Conclusions: Conduction block can occur across discontinuous ablation lines. Moreover, with recovery of conductivity over time, ablation lines with large gaps exhibiting acute conduction block may recover propagation in the gap over time, allowing recurrences of arrhythmias. The ability to see gaps acutely using MRI will allow for targeting these sites for ablation.

Figures

Figure 1

Experimental set-up. Line I: Experimental ablation line with a gap. The gap length was decreased until there was no conduction across the gap. Line II and III: These are ablation lines created prior to Line I and oriented perpendicularly to the experimental line (line I). An electrode array with bipoles (columns A to E) was used to record local electrograms.

Figure 2

Electrograms (EGMs) before and after creation of conduction block across the gap. Letters A to E indicate the electrode column from which the EGMs were recorded. The top tracing is an ECG lead. The left panels show the EGMs before conduction block and the right panels are after conduction block. The top panels were recorded during RV pacing, close to the AV groove. The bottom panels were recorded during LV pacing at the apex. During RV pacing (top left panel) the activation goes sequentially from A to E. After conduction block is achieved (top right panel) the sequence changes and the activation goes from A to B and then after significant delay is recorded at E and then at D and finally at C. Similarly, with LV pacing the sequence changes after conduction block.

Figure 3

Computational model result showing snapshots of transmembrane potential across simulated myocardial tissue. For these models a 5 mm by 5 mm plane of tissue was simulated. Each row represents a different model and the panels in a row show snapshots as time progresses going from left to right. With no ablation line (top row) the depolarization spreads to cover the entire tissue. With an ablation line and a 1.5 mm gap of normal conductivity the depolarization propagates through the gap to cover the entire area. With an ablation line and a 1.3 mm gap of normal conductivity the depolarization does not propagate through the gap area resulting in conduction block.

Figure 4

A plot of the maximum gap length that results in conduction block versus conductivity (normalized to 100%) in the gap area. Panel A is with the ablation line oriented parallel to the fiber direction. Panel B is with the ablation line oriented perpendicular to the fiber.

Figure 5

MRI images of the ablation line. The image on the left was taken using a non-cardiac gating FSE sequence. This is a cross-sectional view (perpendicular to the ablation line) of the ablated lesion (arrow) with three distinct areas. There is an inner area with low intensity (#), middle area with intermediate intensity (&) and an outer rim area with low intensity (*). The image on the right is a section along the length of a linear ablation line with gap taken using a T2-weighted Triple IR FSE sequence. The ablated areas are dark (arrows) and the gap in the middle is bright in appearance.

Figure 6

Macroscopic section and MR image of the ablation line. The top panel shows a gross specimen with a longitudinal section cut along the ablation line. In the middle of the ablation line is the non-conducting gap. The bottom panel is an MR image of an ablation line with gap.

Figure 7

Masson’s trichrome stain of tissue with linear ablations. The left and right panels are from two different specimens. The bottom panels show magnified image of a small area. The ablated area appears as blue and normal myocardium as red. On high magnification (X16), three different areas are noted: inner area of severely damaged coagulation necrosis, middle area of mild damaged coagulation necrosis, and an outermost area of contraction band necrosis.